Kit sets admittedly have their limitations. If you feel that you are technically knowledgeable enough, but are bound by your equipment and not achieving the results you want, maybe it is time to invest on new photography gear. There are many things to consider before adding gadgets to your arsenal, like: What are your needs? What do you want to achieve? Are you using it for your profession, or for hobby? How much are you willing to spend?
Lenses are usually the biggest investment on your set. You may want a recap of our article on camera lenses (http://photography-essentials.blogspot.com/2011/11/camera-lens-type.html) to help you decide which lens you want, depending on the purpose. You can buy the same brand lens as your camera body, but there are also third party lenses which are usually cheaper.